In recent years, camera modules for taking photos have begun to be incorporated in portable terminals such as mobile phones and laptop computers. Downsizing the camera modules is a prerequisite for enhancing the portability of these apparatuses. The camera module operates with an image pickup device such as a CCD (Charged Coupled Device) or a CMOS (Complementary Metal Oxide Semiconductor). Recently, a pixel having the size of approximately a few micrometers has become commercially feasible, and an image pickup device with high resolution and a compact size can now be mass manufactured and marketed. This is accelerating the demand for downsizing of image pick-up lens systems so that they are able to be suitably used with miniaturized image pickup devices. It is also increasing expectations of cost reductions in image pick-up lens systems, commensurate with the lower costs enjoyed by modern image pickup devices. All in all, an image pick-up lens system needs to satisfy the oft-conflicting requirements of compactness, low cost, and excellent optical performance.
Compactness means in particular that a length from a lens edge of the lens system to an image pick-up surface should be as short as possible.
Low cost means in particular that the lens system should include as few lenses as possible; and that the lenses should be able to be formed from a resin or a plastic and be easily assembled.
Excellent optical performance can be classified into the following four main requirements:
First, a high brightness requirement, which means that the lens system should have a small F number (FNo.). Generally, the FNo. should be 2.8 or less.
Second, a wide angle requirement, which means that half of the field of view of the lens system should be 30° or more.
Third, a uniform illumination on the image surface requirement, which means that the lens system has few eclipses and/or narrows down an angle of incidence onto an image pick-up device.
Fourth, a high resolution requirement, which means that the lens system should appropriately correct fundamental aberrations such as spherical aberration, coma aberration, curvature of field, astigmatism, distortion, and chromatic aberration.
In a lens system which satisfies the low cost requirement, a single lens made from a resin or a plastic is desired. However, it is difficult for the single lens system to correct chromatic aberration and achieve excellent optical performance, especially if a wide angle of view such as 70° is desired. Thus, a hybrid diffractive-refractive single lens system is employed to correct chromatic aberration caused by refraction of the lens material. Typical such lens systems can be found in U.S. Pat. No. 6,055,105B1, U.S. Pat. Application Publication No. U.S. 2003/0117709A1 and EP Pat. No. 0819952A2. However, it is still difficult to achieve excellent optical performance in a wide angle of view. For example, distortion, field curvature and astigmatism of the system cannot be optimally corrected. Thus, the hybrid diffractive-refractive single lens system can generally only be used in a low-resolution image pickup device such as a CMOS.
In a lens system which satisfies the excellent optical performance requirement, two or even more lenses are desired. A typical two-lens system can be found in U.S. Pat. Application Publication Nos. 2003/0117723 and 2004/0036983, and EP Pat. No. 1357414A1. In order to correct chromatic aberration, the two lenses of the system must be made from different materials, with the lenses having a relatively large difference being their respective Abbe constants. Because there are only a few varieties of plastic and resin materials which can be suitably used to make lenses, even if the two lenses are made from a different plastic or resin material, the range of variation of optical properties of the two lenses is limited. This makes it difficult to effectively correct chromatic aberration. Therefore, in most two-lens systems which have excellent optical performance, at least one of the lenses is made from optical glass. As a result, such systems generally yield limited cost efficiency and lightness in weight.
With further developments in lens molding technology, it is now possible to mass manufacture plastic lenses with aspheric surfaces and plastic lenses with diffractive surfaces. Therefore, hybrid diffractive-refractive two-lens systems having the lenses made from a plastic or a resin material are being developed. Such lenses can correct chromatic aberration caused by refraction of the lens material, as well as enjoy reduced manufacturing costs. A well-known hybrid diffractive-refractive two-lens system is the retro-focus type lens system. The lens system comprises, from an object side to an image side, a first lens having negative refracting power, a stop, and a second lens having positive refracting power. A typical such lens system can be found in U.S. Pat. No. 6,055,105 and in WO App. No. 96/17265. The lens system helps correct wide angle of view aberrations. However, a shutter is positioned between the second lens and the image side, which adds to the distance between the second lens and the image side. Thus, the compactness of the lens system is limited.
Another important consideration is that plastic and resin materials are prone to absorb water. For example, the water absorbency of polymethyl methacrylate (PMMA) is 1.5%, and the water absorbency of polycarbonate (PC) is 0.4%. Among the plastic or resin materials which can be suitably used to make lenses, only zeonex materials (polyolefin resins or cyclo-olefin polymers) have relatively low water absorbency. The water absorbencies are less than 0.01%. Zeonex materials are available from the Japanese Zeon Corporation. Therefore unless a non-glass lens is made from a zeonex material, it is liable to absorb water and deform. As a result, the optical performance of the lens system is diminished.
Therefore, a low cost image pick-up lens system which has a compact configuration and excellent optical performance is desired.